Tony Blair meets British troops at Camp Bastion in Afghanistan in 2006, when Blair was prime minister. Photograph: Stefan Rousseau/PA

Tony Blair, who led the UK into wars in Iraq and Afghanistan, is to donate the proceeds from his forthcoming memoir to the Royal British Legion.

A spokesman for the former prime minister, who is understood to have already received a £4.6m advance for A Journey, said he would hand over all the money he makes – including the advance – "as a way of marking the enormous sacrifice [the armed forces] make for the security of our people and the world".

Blair's spokesman added: "In making this decision, Tony Blair recognises the courage and sacrifice the armed forces demonstrate day in, day out. As prime minister he witnessed that for himself in Iraq, Afghanistan, Northern Ireland, Sierra Leone and Kosovo. This is his way of honouring their courage and sacrifice."

The book is expected to generate huge interest when it goes on sale on 1 September. Translation rights have been sold in 14 territories.

"The proceeds will go to the Royal British Legion's Battle Back challenge centre, a project that will provide state-of-the-art rehabilitation services for seriously injured troops returning from the frontline," the spokesman said. "We have been consulting with a number of people and organisations to decide the best support he can give. [This] is one project consistently highlighted."

It is understood that negotiations about the donation have been going on for several months.

Chris Simpkins, the charity's director general, said he was delighted to accept "this very generous" offer, which he said would help the organisation to hit its £12m fundraising target.

"The culture of the centre will very much be about what users of the service can do rather than what they can't, but some of the servicemen and women are likely to need the Legion's support for the rest of their lives. Mr Blair's generosity is much appreciated and will help us to make a real and lasting difference to the lives of hundreds of injured personnel."

Blair's decision to invade Iraq in 2003 prompted huge anti-war protests and led to splits within the Labour party amid claims the war was illegal.

Today his spokesman said the former prime minister had always been committed to recognising the role played by the UK's armed forces.

"As Tony Blair said to the House of Commons on his last day in office, 'I believe that they [the armed forces] are fighting for the security of this country and the wider world against people who would destroy our way of life. But whatever view people take of my decisions, I think that there is only one view to take of them: They are the bravest and the best.'"

However, anti-war activists who are preparing to protest next month in London at a planned appearance by Blair to sign copies of his book said that his decision to give the money to the British Legion proved that he had a "guilty conscience".

A spokesman for the Stop The War Coalition said: "Tony Blair's decision to give the money made from his memoirs to the British Legion is welcome if it means that some of those who have suffered as a direct consequence of his criminal wars will benefit."

"But no proportion of Tony Blair's massive and ill-gotten fortune can buy him innocence or forgiveness. He took this country to war on a series of lies against the best legal advice and in defiance of majority opinion."

"The wars in Iraq and Afghanistan have resulted in the pointless death of hundreds of British soldiers and hundreds and thousands of innocent civilians."